1. Field of the invention
The present invention relates to a placement structure for a monochromatic image forming apparatus or a full-color image forming apparatus employing electrophotography.
2. Related Background Art
In the field of image forming apparatuses employing electrophotography such as full-color printers, a smaller-sized printer with a lower price and higher image quality is required by the market, in particular, as more and more personal computers are being generally used.
FIGS. 5A and 5B are schematic section diagrams of an indirect-transfer-type, full color image forming apparatus that employs electrophotography.
In FIGS. 5A and 5B, reference numeral 11 denotes paper feeding means for separating stacked paper sheets and feeding them; reference numeral 12 denotes a registration roller for detecting the leading edge of a paper sheet and conveying the paper sheet in synchronization with an image signal; reference numeral 13 denotes a transfer belt for forming an intermediate image; reference numeral 14 denotes a developing device having four photosensitive drums 19; reference numeral 15 denotes exposure means for forming a latent image on each photosensitive drum 19; reference numeral 16 denotes first-transfer means for transferring a toner image onto the transfer belt 13 by reverse biasing; reference numeral 17 denotes second-transfer means for transferring the toner image from the transfer belt 13 onto a paper sheet by reverse biasing; and reference numeral 18 denotes a fixing device for fixing the toner as a permanent image on the paper sheet by heating and pressing.
In the indirect-transfer-type, full-color image forming apparatus having such an arrangement, a latent image on each photosensitive drum 19 is visualized as a toner image on the developing device 14; the toner image is first-transferred on the transfer belt 13, and the toner image is then second-transferred onto a paper sheet by reverse biasing from the backside of the paper sheet, thereby forming a desired full-color image.
A function unit, which is constituted of the above described paper feeding means 11, registration roller 12, transfer belt 13, developing device 14, exposure means 15, first-transfer means, second transfer means 17, and fixing device 18, is fixed to a main frame 6. The main frame 6 is then supported by four placement legs 21 (only two of them shown) provided at front and rear, left and right positions.
A tandem-type, multicolor image forming apparatus, which has a plurality of photosensitive drums 19 arranged in line, may be an image forming apparatus of lateral arrangement in which a plurality of photosensitive drums 19 are arranged horizontally as shown in FIG. 5A, or an image forming apparatus of vertical arrangement in which a plurality of photosensitive drums 19 are arranged vertically as shown in FIG. 5B. An image forming apparatus having a lateral arrangement shown in FIG. 5A has a smaller height, but is difficult in replacement of consumables in the developing device 14 or the like. On the other hand, an image forming apparatus having a vertical arrangement, as shown in FIG. 5B, allows easy replacement of consumables but has a greater height.
Such a tandem-type multicolor image forming apparatus requires high positional accuracy between photosensitive drums that form respective colors. However, when the multicolor image forming apparatus is placed on an uneven surface, for example, on an area where it is placed with four placement points with only one point being different in a height level than the other points, the following problem arises. Since the straight line connecting a point and another point of different height levels and the straight line connecting the two other points are not parallel with each other, a force results that deforms the main body of the image forming apparatus. The occurrence of deformation of the main body of the image forming apparatus changes the positional relationship between the photosensitive drums and scanner as the exposure means for each color, depending on the strength of the bottom plate of the main body of the image forming apparatus, leading to misregistration in images.
Accordingly, in the case where tension is given to an endless belt to move according to the frame of the main body of the image forming apparatus, the deformation of the frame of the main body of the image forming apparatus may increase the displacement force on the endless belt. As a result, the increase in the displacement force on the endless belt may cause the deterioration or breakage of an end portion of the belt, affecting the life of the endless belt. The endless belt may be, for example, an electrostatic transfer belt that contacts with a plurality of photosensitive drums as image bearing members and conveys transfer material attracted electrostatically thereon to a fixing device consisting of a fixing roller and a pressing roller, or an intermediate transfer belt that contacts with a plurality of image bearing members, sequentially transfers and overlays images onto the belt, and transfers all images on the belt onto the transfer material at a time.
Therefore, there are methods for preventing the above problems by reducing the deformation of the main frame, as follows. A first method is to substantially increase the rigidity of the main frame so that the main frame cannot be deformed even when the main body of the image forming apparatus is placed on the surface mentioned above. A second method is to substantially increase the rigidity of the bottom plate against deformation so that the unevenness of the placement surface cannot cause the main frame to be deformed. A third method is to use only three placement points (Japanese Patent Application Laid-Open No. 2000-330351). In the case where only three placement points are used, a plane is defined by the three points, and therefore the main frame is not deformed even when the main body of the image forming apparatus is placed on an uneven surface.
In the first and second methods, to increase the rigidity of components of the main frame or the bottom plate against deformation, the thickness of the bottom plate may be increased or the section area thereof may be increased with a drawing shape or the like. This results in the heavy weight of the main body and the increased cost for parts. Further, the use of a drawing shape for improving the rigidity may badly affect the flatness and the dimension accuracy, resulting in the lowered accuracy in mounted position of the photosensitive drums or scanner. Therefore, this presents a significant problem in developing a smaller-sized printer with a lower price and higher quality image, for which the market has a strong demand.
Regarding the reduction of the deformation of the main frame, the most advantageous method for providing a smaller-sized printer with a lower price and higher quality image is the above-mentioned, third method that uses only three placement points, but the method has a significant problem as described below.
FIGS. 7A and 7B shows side section diagrams of a typical arrangement of an image forming apparatus in which a paper feeding cassette 22 for stacking recording paper sheets is provided at the bottom thereof; an image forming section is provided above the paper feeding cassette 22; a paper discharge tray 23 is provided at the top thereof; and a recording paper sheet is conveyed in the direction of the arrow so that operations including paper feeding, image formation and paper discharge are performed. Here, the shown dimension X, which may be a depth or width of the product, is designed to be as small as possible to provide a smaller-sized image forming apparatus.
To achieve such a smaller-sized printer for which the market has a strong demand, it is a common practice that recording paper sheets of A4, B5 or letter size, which are more frequently used as transfer material, are accommodated in a size within the shown dimension X as shown in FIG. 7A, while larger-sized paper sheets such as those of A3 or legal-size, which are less frequently used as transfer material, are accommodated in the paper feeding cassette 22 that has a larger size than the shown dimension X, as shown in FIG. 7B. Thus, when using a recording paper of (or smaller than) A4, B5 or letter size, which are frequently used, the placement area can be within the shown dimension X, thereby allowing effective use of space. When the placement area is increased by the shown dimension Y, larger-sized recording paper can be used. Therefore, such a type of arrangement in which larger-sized recording paper is used while exceeding the normal placement area is very advantageous for users.
In order to achieve both the arrangements: the arrangement in which only three placement points as described above is defined to reduce the deformation of the main frame, and the arrangement of the type in which larger-sized recording paper is used while exceeding the normal placement area, it is required that three placement points A, B and C are defined symmetrically with respect to the center of image, and in particular the placement point C is defined under the paper feeding cassette 22 as shown FIG. 8A, or that three placement points A′, B′ and C′ are defined non-symmetrically with respect to the center of image as shown in FIG. 8B.
However, even if the placement point C is provided under the paper feeding cassette 22 as shown in FIG. 8A, using only the space under the paper feeding cassette 22 is not enough since substantial rigidity along edge line a is required for supporting the body. Therefore, to secure the rigidity along edge line a, an expensive member having large section area must be applied to increase the rigidity of the bottom plate. Moreover, expanding the space under the cassette 22 interferes with the object of using the arrangement with only three placement points to provide a smaller-sized, lower-price apparatus.
When using the arrangement in which the main frame is non-symmetrically supported as shown in FIG. 8B, placement legs cannot be provided on the front side of the apparatus, which is a side for taking out the paper feeding cassette 22. The front side of the apparatus can be subject to a load from a user who accesses the operation section or replaces consumables. This causes the non-symmetrical deformation of the main frame, which directly leads to misregistration, the increase in displacement force on the endless belt, and inclined conveyance of recording paper. Therefore, this arrangement is not feasible.
Furthermore, in the three-placement point case, each point bears a heavier load, and, thus, the bottom place must be reinforced more than otherwise required.